1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-228510, filed on Sep. 4, 2007, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to an optical communication system in which an optical pulse is phase- and intensity-modulated and then transmitted and, more particularly, to an optical transmitter and a method for controlling a composite modulator used in the optical transmitter.
2. Description of the Related Art
The Internet, which is growing rapidly, is convenient, but, in actual fact, there is great apprehension about its security. The necessity for high-degree cryptographic technologies is ever increasing to maintain secrecy in communication. The cryptographic methods that are currently used in general are broadly classified into secret key cryptography, such as Data Encryption Standard (DES) and Triple DES, and public key cryptography, such as Rivest Shamir Adleman (RAS) and elliptic curve cryptography (ECC). However, these are cryptographic communication methods that guarantee the security of communication based on computational complexity. Ciphers according to these methods are in constant jeopardy of being broken with the advent of vast computing power or a cryptanalysis algorithm. In such a background, quantum key distribution (QKD) is attracting attention as a cryptographic key distribution technique that makes eavesdropping absolutely impossible.
In QKD, a photon is generally used as a medium of communication, and information is transmitted by being superposed on the quantum state of a photon. If an eavesdropper existing on a transmission path eavesdrops on the information by, for example, tapping a photon being transmitted, it is impossible to perfectly return the quantum state of the once-observed photon to the state before observation, according to Heisenberg's uncertainty principle. This causes a change in the statistics of the reception data detected by an authorized receiver. The detected change allows the receiver to detect the presence of the eavesdropper on the transmission path.
In the case of a quantum key distribution method utilizing the phase of a photon, a sender and a receiver (hereinafter, referred to as “Alice” and “Bob,” respectively) organize an optical interferometer. Alice and Bob individually modulate the phase of each photon at random. Based on the depth difference between the modulated phases, an output of “0,” “1,” or “indeterminate” can be obtained. Thereafter, the conditions used when output data were measured are partly compared between Alice and Bob, whereby a sequence of random numbers can be finally shared between Alice and Bob. The sequence of random numbers shared here includes errors caused by external disturbances such as photon-receiver noises, noises leaking from a classical-channel signal, and noises caused depending on the precision of the interferometer. In addition, it should be thought that the sequence of random numbers also includes errors caused by an act of eavesdropping committed by an eavesdropper (hereinafter, referred to as “Eve”). Therefore, to obtain a final cryptographic key, Alice and Bob carry out error correction for eliminating the errors in the shared sequence of random numbers and privacy amplification for sifting out the information that Eve may possess.
For the configuration most suitable for practical use, a plug and play system is frequently used, which is shown in Ribordy, G., Gautier, J.-D., Gisin, N., Guinnard, O., and Zbinden, H. (“Automated ‘plug & play’ quantum key distribution,” Electronics Letters, Vol. 34, No. 22, pp. 2116-2117), and others. In a plug and play system, an optical interferometer of a round-trip type is constructed so that a single interferometer functions as an interferometer for temporally separating a photon pulse and an interferometer for combining the temporally separated photon pulse pair again. Accordingly, this system has the merit that high-precision interference can be achieved if the optical path difference made in the interferometer is kept constant for a period of time longer than the duration of a round trip of a photon pulse.
However, QKD methods of such a round-trip type are unsuitable to increase the key sharing rate because phase modulators for modulating the phase of a photon pulse need to be used in two ways. In addition, there is also a demerit that the signal-to-noise ratio of a photon signal is degraded because the occurrence of backscattering light in a transmission path is inevitable.
On the other hand, according to QKD methods of a one-way type, a sender and a receiver have different asymmetric interferometers respectively. That is, the interferometer for temporally separating a photon pulse and the interferometer for combining the temporally separated photon pulse pair again are placed at distant locations. Accordingly, some techniques are needed to keep the optical path differences made in the multiple interferometers precisely equal. For example, Yuan, Z. L. and Shields, A. J. (“Continuous operation of a one-way quantum key distribution system over installed telecom fibre,” Optics Express, Vol. 13, pp. 660-665), discloses a system in which a fiber stretcher is provided to one of the paths of the asymmetric interferometer on Bob's side and the length difference between the optical paths of the asymmetric interferometer is adjusted by controlling the fiber stretcher while monitoring interference characteristics.
However, since optical fiber has a linear expansion coefficient ranging from 10−6 to 10−5/K, an optical fiber line with a length of 100 cm (corresponding to a delay of 5 ns) extends/contracts approximately 100 to 1000 nm, with a 0.1-degree change in temperature. Since an optical signal to be used in QKD and general optical communication has a wavelength of 1550 nm, if an asymmetric interferometer having a delay of several nanoseconds is used, stable interference characteristics cannot be obtained unless temperature control of the entire optical fiber for delay is performed with a granularity smaller than 0.01 degrees. According to Bonfrate, G., Harlow, M., Ford, C., Maxwell, G., and Townsend, P. D. (“Asymmetric Mach-Zehnder germano-silicate channel waveguide interferometers for quantum cryptography systems,” Electronics Letters, Vol. 37, No. 13, pp. 846-847), optical paths are installed in a small area by using planar lightwave circuit (PLC) technology, whereby temperature control is facilitated.
As described above, techniques have been gradually established for stabilizing the relative delay amounts in multiple interferometers, which has been a challenge in implementing one-way QKD. In response to this trend, one-way QKD has begun to develop in various ways in recent years. For example, Nambu, Y., Yoshino, K., and Tomita, A. (“One-Way Quantum Key Distribution System Based on Planar Lightwave Circuits,” Japanese Journal of Applied Physics, Vol. 45, pp. 5344-5348) proposes a first one-way QKD system using a general two-input, two-output Mach-Zehnder interferometer. A second one-way QKD system using no phase modulator is also proposed in Nambu, Y., Yoshino, K., and Tomita, A. (“Quantum key distribution systems without optical switching using planar lightwave circuit,” The 8th International Conference on Quantum Communication, Measurement and Computing, pp. 2-31). Both of these schemes are embodiments of the BB84 protocol using four quantum states (see Bennett, C. H. and Brassard, G., “QUANTUM CRYPTOGRAPHY: PUBLIC KEY DISTRIBUTION AND COIN TOSSING” in Proceedings of the IEEE International Conference on Computers, Systems, and Signal Processing, Bangalore (1984), pp. 175-179). Next, the first and second one-way QKD systems will be described.
FIG. 1A is a block diagram schematically showing the first one-way QKD system. According to the scheme shown in FIG. 1A (hereinafter, referred to as “scheme A”), four phase states are used.
First, an optical pulse generated by a light source 11 on Alice's side is temporally separated into a temporally separated pulse pair (hereinafter, referred to as double pulses) by a PLC asymmetric Mach-Zehnder (AMZ) interferometer 12, and the phase difference between these double pulses is modulated into any one of four states (0, π/2, π, and 3π/2) by phase modulators 13 and 14. Hereinafter, a coding set of phase differences of 0 and π will be referred to as “X basis,” and a coding set of phase differences of π/2 and 3π/2 will be referred to as “Y basis.”
In Bob, the phase difference between the double pulses received from Alice is modulated again into 0 or −π/2 by a phase modulator 16. Thereafter, the result of interference of the double pulses, which are combined by a PLC AMZ interferometer 17, is detected by one of photon detectors APD1 and APD2.
In the scheme A, there are two methods to carry out four-state phase modulation on Alice's side: a method by which a single phase modulator is driven with four values, and a method by which two phase modulators are driven each with two values. The former method, by which a single phase modulator is driven with four values, has the merit of reducing the space and power consumption of the sender because only one phase modulator is required. However, this method also has a demerit as follows. In a high-speed transmission system in which the system repetition rate exceeds 1 GHz, it is difficult to set each of the levels of four-valued driving signals with high precision, and each of the phase states of 0, π/2, π, and 3π/2 is deteriorated in precision, resulting in the key generation efficiency being degraded. On the other hand, according to the latter method, by which two phase modulators are driven each with two values, the four phase states can be generated with high precision by individually controlling the amplitude of each of the two-valued signals.
FIG. 1B is a block diagram schematically showing the second one-way QKD system. In the scheme shown in FIG. 1B (hereinafter, referred to as “scheme B”), two phase states and two time states are used.
First, on Alice's side, using a four-input, two-output PLC AMZ interferometer 21, optical pulses from light sources LD1 to LD4 are input to four input ports respectively. In the case of an optical pulse input from the light source LD1, since the optical pulse travels along only a long path of the interferometer 21, only a single pulse temporally delayed is sent out to a transmission line. In the case of an optical pulse input from the light source LD4, since the optical pulse travels along only a short path of the interferometer 21, only a single pulse relatively advanced is sent out to the transmission line. In the case of optical pulses input from the light sources LD2 and LD3, the X or Y basis can be generated depending on the phase difference between the optical pulses traveling along the long and short paths of the PLC AMZ interferometer 21. On Bob's side, each basis is decoded by using a two-input, four-output PLC AMZ interferometer 23 and detected by using one of photon detectors APDZ1, APD1, APD2, and APDZ2.
Hereinafter, a coding set corresponding to the case where only one of the double pulses exists as in the case of using the light source LD1 or LD4 will be referred to as “Z-basis.” Note that the optical intensity of each of the double pulses made when the X or Y basis is selected is half the optical intensity made when the Z basis is selected because the total sum of the optical intensities of the double pulses needs to be equal to the optical intensity made when the Z basis is selected.
That is, according to the scheme B, a selection is made from the four light sources LD1 to LD4 to generate an optical pulse on Alice's side, and a photon is detected by using one of the photon detectors APDZ1, APD1, APD2, and APDZ2 on Bob's side, whereby it is possible to determine a bit and basis at the same time.
With respect to the configuration of the receiver, the schemes A and B each has a merit and demerit. The scheme B has the merit that it is possible to increase the distance and speed by an amount equivalent to a loss caused by a phase modulator because no phase modulator is required on Bob's side, but has the demerit that the power consumption is large because four photon detectors are required. On the other hand, the scheme A has a merit in terms of power consumption because only two photon detectors are used, but has the demerit of being unsuitable to increase the distance and speed due to the loss attributable to the phase modulator on Bob's side. Accordingly, considering the merits and demerits of the schemes A and B, a desirable sender is a transmitter that is applicable to both schemes. However, such a transmitter that can be applied to both the schemes A and B has not hitherto been proposed.
Moreover, in the scheme B shown in FIG. 1B, four light sources LD1 to LD4 are required. However, as a practical problem, it is extremely difficult to make the spectra of light output from the four light sources perfectly match. If the deviation is large, Eve can correctly determine a quantum state being transmitted by measuring wavelength deviation and thus can duplicate the quantum state without leaving any trace of eavesdropping.
To make the sender configuration according to the scheme B shown in FIG. 1B applicable to the scheme A shown in FIG. 1A, if multiple modulators are simply placed at an output of the PLC AMZ interferometer 21 on Alice's side, four modulators are required. This will be described with reference to FIGS. 2A and 2B.
FIG. 2A is a conceptual diagram collectively showing the modulations that are required to prepare the X, Y and Z bases. FIG. 2B is a signal constellation diagram showing the signal points according to the modulations shown in FIG. 2A. Referring to FIG. 2A, output from the two-input, two-output PLC AMZ interferometer 12 according to the scheme A are double pulses of the same intensity. When Z-basis modulation is performed, one of these double pulses is completely extinguished by using an intensity modulator. When X- or Y-basis modulation is performed, the intensities of both pulses are reduced by half by using an intensity modulator, and at the same time, it is necessary to produce a phase difference between the double pulses by using phase modulators. That is, four-state phase modulation (in steps of π/2) is needed to correspond with the receiver according to the scheme A, and three-state (0, ½, 1) intensity modulation is needed to correspond also with the receiver according to the scheme B. In FIG. 2B, required signal points are plotted on an IQ plane.
Referring to FIG. 2B, the phase of a point A does not need to be zero because the phase state of a Z-basis optical pulse is not restricted. Therefore, the point A can be any point on the same-intensity circle with as long a radius as the distance from the original point of the IQ plane to the point A. To realize such modulation, it is possible to utilize a modulation method described in Hayase, S., Kikuchi, N., Sekine, K., and Sasaki, S. (“Proposal of 8-state per Symbol (Binary ASK and QPSK) 30-Gbit/s Optical Modulation/Demodulation Scheme,” ECOC 2003, Th.2.6.4). Hayase et al. discloses a modulator configured by using three optical modulators in total (two phase modulators and one intensity modulator) to implement 8-state (four values in phase and two values in intensity) amplitude phase shift keying (APSK) modulation.
FIG. 3A is a block diagram depicting the configuration of an ASK-QPSK transmitter described in Hayase et al. and FIG. 3B is a signal constellation diagram showing the signal points according to this transmitter.
Three modulators in total (a phase modulator 32 for (0, π) modulation, a phase modulator 33 for (0, π/2) modulation, and an intensity modulator 34 for (½, 1) modulation) are connected in cascade on the output side of a light source 31 as shown in FIG. 3A, whereby the required signal constellation on the IQ plane as shown in FIG. 3B can be prepared. However, when comparing the signal constellation in FIG. 3B with the signal constellation in FIG. 2B, there is no signal point of an intensity of 0 in FIG. 3B. Therefore, to supply this signal point, it is necessary to add one more intensity modulator. Specifically, an intensity modulator for (0, 1) modulation needs to be connected in cascade on the output side of the light source 31 in addition to the phase modulator 32 for (0, π) modulation, phase modulator 33 for (0, π/2) modulation, and intensity modulator 34 for (½, 1) modulation.
As described above, if an attempt is made to obtain a signal constellation as shown in FIG. 2B by utilizing the configuration of the transmitter according to Hayase et al., four optical modulators are required in total. It is desirable that the number of modulators to be used be as smalls as possible from the viewpoint of power consumption and space saving.